image: Split host code out into its own file

To avoid having #ifdefs in a few functions which are completely different
in the board and host code, create a new image-host.c file.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/Makefile b/tools/Makefile
index 4a86321..999fd46 100644
--- a/tools/Makefile
+++ b/tools/Makefile
@@ -113,6 +113,7 @@
 			lib/fdtdec_common.o \
 			lib/fdtdec.o \
 			common/image.o \
+			common/image-host.o \
 			imagetool.o \
 			imximage.o \
 			imx8image.o \